Hydralazine, the prototypic drug that induces systemic lupus erythematosus, reacts at therapeutic concentrations with labeled thymidine to form at least four labeled products. Formation of these products is markedly enhanced by exposure to ultraviolet light. At higher concentrations, hydralazine reacts with labeled deoxycytidine to form at least three labeled products. Ultraviolet light also enhances formation of these products. Hydralazine reacts with thymine and deoxycytosine residues in DNA resulting in their modification and excision. This reaction is also markedly enhanced by exposure to ultraviolet light. Hydralazine-pyrimidine reaction products are being characterized. Sera collected from rabbits immunized with hydralazine and from patients with hydralazine-induced antinuclear antibodies and hydralazine-induced LE are being used to study antibody binding to modified nucleosides. Chlorpromazine and procainamide interactions with nucleic acids will be further examined.